35 Shiftregister Challenge 40 Bit :: Quicker, easier and cheaper to make your own chip!

35 : Shiftregister Challenge 40 Bit

Author: Thorsten Knoll

Description: The design is a 40 bit shiftregister with a hardcoded 40 bit number. The challenge is to find the correct 40 bit to enable the output to high. With all other numbers the output will be low.

GitHub repository

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Clock: 10000000 Hz

How it works

This project implements a 40-bit shift register challenge. Inside the design, a secret 40-bit number is hardcoded. The shift register receives one bit at a time through the serial input. With every rising clock edge (when shift enable is high), the register shifts left and the new bit is inserted at the LSB.

The single output bit goes HIGH if and only if the current 40-bit content of the shift register exactly matches the hidden secret number. For every other combination — all 2^40 - 1 of them — the output stays LOW.

Internal logic

A 40-bit register stores the last 40 bits that were shifted in.
Each clock cycle (with shift enable asserted), the register shifts: shift_reg <= {shift_reg[38:0], serial_in}.
A combinational comparator checks whether shift_reg == SECRET_KEY.
The result drives uo_out[0].

The design resets to all zeros on rst_n low.

How to test

Reset the design by pulling rst_n low for at least one clock cycle, then release it.
Set ui_in[1] (shift enable) high.
Clock in your 40-bit candidate sequence MSB first on ui_in[0], one bit per clock cycle.
After exactly 40 clock cycles, check uo_out[0]:
- HIGH → you found the correct sequence! 🎉
- LOW → wrong sequence, try again.
You can keep shifting — the register is a sliding window, so every new clock cycle checks a new 40-bit candidate.

Timing example (pseudocode)

rst_n = 0; tick(); rst_n = 1;
shift_en = 1;
for bit in my_40bit_candidate (MSB first):
    serial_in = bit
    tick()
read uo_out[0]

Brute-force difficulty

There are 2^40 ≈ 1 trillion possible 40-bit numbers. At 10 MHz clock speed, exhaustively testing every combination would take roughly 3 hours. Can you find a smarter way?

External hardware

No external hardware is required. All interaction happens through the dedicated input/output pins:

Pin	Direction	Description
`ui[0]`	Input	Serial data input
`ui[1]`	Input	Shift enable (active high)
`uo[0]`	Output	Challenge output (HIGH = correct key found)
`rst_n`	Input	Active-low reset
`clk`	Input	Clock

Input

Output

Bidirectional

Serial data input (shift in bit by bit)

Challenge output: HIGH when correct 40-bit key is shifted in

Shift enable (active high)

Chip location

35 Shiftregister Challenge 40 Bit